The present application relates generally to circuit board manufacturing. More specifically, the present application is directed to signal via positioning in a multi-layer circuit board.
Printed circuit boards (“PCB”) are generally fabricated from a plurality of laminated layers. Each of the layers typically includes a core fabricated from an insulating material, such as FR-4, epoxy glass, polyester or synthetic resin bonded paper, for example. Typically, a copper layer is bonded to one or both sides of the core. Circuits or “traces” are formed on the copper by applying a mask and removing unneeded copper. The individual layers are then laminated together to form the PCB.
Energy is typically transferred between layers of the PCB using power vias, signal vias, and ground vias. Signal vias are typically installed through all layers of a PCB even though the signal vias may only form an electrical connection between two of the layers. A portion of a signal via that extends through layers of the PCB beyond the points of electrical connection is referred to as a “stub”. For high-speed signals, if stubs are not removed, sharp resonances can appear as an insertion loss, particularly in a differential signal pair.
Stubs can be removed by back-drilling the unused portion of the signal vias out of the PCB during manufacturing. However, the ability to back-drill signal vias is constrained by a minimum drilling distance defined between the signal vias. The potential for damaging the PCB increases considerably if the minimum drilling distance is violated. This minimum drilling distance effectively constrains the minimum pitch (i.e., center-to-center distance) that can be supported, and thus, limits signal density under a chip or module.